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Windows Exchange UMWorkerProcess File Drops Indicating CVE-2021-26858 Exploitation

Alerts on Exchange Unified Messaging (UMWorkerProcess.exe) creating unusual files, excluding common benign names consistent with CVE-2021-26858 activity.

FreeReviewedSigma · High · v5
Product
windows
Category
file_event
Author
Bhabesh Raj (SigmaHQ), DRL 1.1
Published
2021-03-03
Updated
2026-07-31
AnalysisDetection

ATT&CK techniques

Execution

  1. Recon

  2. Resource Dev

  3. Initial Access

  4. Execution

  5. Persistence

  6. Priv Esc

  7. Defense Evasion

  8. Cred Access

  9. Discovery

  10. Lateral Movement

  11. Collection

  12. C2

  13. Exfiltration

  14. Impact

What it detects

This rule flags file creation events where the process name ends with UMWorkerProcess.exe, which is associated with Exchange Unified Messaging. It filters out specific benign-looking target filenames and alerts on other non-standard files that could represent dropped web shells or other attacker content following exploitation. The detection relies on Windows file event telemetry that captures the writing process and the target filename.

Reporting behind it

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Malicious Foomatic-Rip Filter Spawning Shell via CUPS Exploitation
Suspicious Child Process Spawned by WinRAR via Process Creation
Malicious Script Execution from WinRAR Extraction Directory via CVE-2023-38831
Windows Exchange UMWorkerProcess File Drops Indicating CVE-2021-26858 Exploitation
Pivot detection · T1203 · 9 related

Changelog

v5
  1. v5
    Candidate ingested via manual entry.2026-07-31
  2. v4
    Candidate ingested via manual entry.2026-07-31
  3. v3
    Candidate ingested via manual entry.2026-07-31
  4. v2
    Candidate ingested via manual entry.2026-07-31
  5. v1
    No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.